1. Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA
2. CARS, University of Chicago, Chicago, Illinois 60637, USA

Correspondence to: Sang-Heon Shim¹ Correspondence and requests for materials should be addressed to S.-H.S. (e-mail: Email: sangshim@princeton.edu).

The 660-km seismic discontinuity in the Earth's mantle has long been identified with the transformation of (Mg,Fe)₂SiO₄ from -spinel (ringwoodite) to (Mg,Fe)SiO₃-perovskite and (Mg,Fe)O-magnesiowüstite. This has been based on experimental studies of materials quenched from high pressure and temperature^{1, 2, 3}, which have shown that the transformation is consistent with the seismically observed sharpness and the depth of the discontinuity at expected mantle temperatures⁴. But the first in situ examination of this phase transformation in Mg₂SiO₄ using a multi-anvil press⁵ indicated that the transformation occurs at a pressure about 2 GPa lower than previously thought (equivalent to 600 km depth) and hence that it may not be associated with the 660-km discontinuity. Here we report the results of an in situ study of Mg₂SiO₄ at pressures of 20–36 GPa using a combination of double-sided laser-heating and synchrotron X-ray diffraction in a diamond-anvil cell. The phase transformation from -Mg₂SiO₄ to MgSiO₃-perovskite and MgO (periclase) is readily observed in both the forward and reverse directions. In contrast to the in situ multi-anvil-press study⁵, we find that the pressure and temperature of the post-spinel transformation in Mg₂SiO₄ is consistent with seismic observations^{4, 6} for the 660-km discontinuity.